You have quite a project boat. In addition to missing the boom, you need to build a rudder. That in and of its self is a big project. You have the innards, i.e., the rudder post and the flange, but you will have to mold a rudder shell out of fiberglass and then mount it to the rudder post and join the 2 halves. That said, it can be done.
In order to answer your questions and for you to ask well formed questions, I think it will necessary for you to understand boat construction. There are reasons boats are built the way they are. I found this book on Amazon that might be helpful for you to gain a basic and fundamental understanding of how and why boats are built the way they are. I haven't read it, so I can't comment on its utility, used copies were available for under $30,
Fiberglass Boat Building for Amateurs